On Wed, Feb 2, 2022 at 2:52 AM Ravi Bangoria <ravi.bangoria@xxxxxxx> wrote: > > Perf counter may overcount for a list of Retire Based Events. Implement > workaround for Zen3 Family 19 Model 00-0F processors as suggested in > Revision Guide[1]: > > To count the non-FP affected PMC events correctly: > o Use Core::X86::Msr::PERF_CTL2 to count the events, and > o Program Core::X86::Msr::PERF_CTL2[43] to 1b, and > o Program Core::X86::Msr::PERF_CTL2[20] to 0b. > > Note that the specified workaround applies only to counting events and > not to sampling events. Thus sampling event will continue functioning > as is. > > Although the issue exists on all previous Zen revisions, the workaround > is different and thus not included in this patch. > > This patch needs Like's patch[2] to make it work on kvm guest. IIUC, this patch along with Like's patch actually breaks PMU virtualization for a kvm guest. Suppose I have some code which counts event 0xC2 [Retired Branch Instructions] on PMC0 and event 0xC4 [Retired Taken Branch Instructions] on PMC1. I then divide PMC1 by PMC0 to see what percentage of my branch instructions are taken. On hardware that suffers from erratum 1292, both counters may overcount, but if the inaccuracy is small, then my final result may still be fairly close to reality. With these patches, if I run that same code in a kvm guest, it looks like one of those events will be counted on PMC2 and the other won't be counted at all. So, when I calculate the percentage of branch instructions taken, I either get 0 or infinity. > [1] https://bugzilla.kernel.org/attachment.cgi?id=298241 > [2] https://lore.kernel.org/lkml/20220117055703.52020-1-likexu@xxxxxxxxxxx > > Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
